Ulva rigida is a bright green seaweed with a variety of growth forms. The two cell layer thick, sheet-like blades can grow as tiny blades forming carpet-like turfs, or as clumps of several larger blades, or solitarily. In its largest form, Ulva rigida can reach up to 10 centimeters in height, but size and blade shape are both highly variable. Blades may be flat or ruffled, sometimes bearing many small holes (perforations) and sometimes not. However, the defining characteristics of Ulva rigida are common among all its growth forms: the two cell layers of the blades are easily separable, like a deflated balloon, and the holdfasts are composed of many small, tough, dark rhizhoids that are absent in other Ulva species. Ulva rigida is common in intertidal and shallow subtidal waters, often found in tidepools. This seaweed is able to withstand high variation in salinity and water chemistry and actually seems to prefer areas with freshwater input.

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Original publication: Agardh, C.A. (1823). Species algarum rite cognitae, cum synonymis, differentiis specificis et descriptionibus succinctis. Volumen primum pars posterior. pp. [vii-viii], [399]-531. Lundae [Lund]: ex officina Berlingiana.

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Digenetic lifecycle.

Ulva rigida, like all species of Ulva, is an early colonizing, fast-growing, opportunistic seaweed. Ulva species are often the first species to colonize cleared substrate space and their high growth rate makes it possible for them to remain a top species in near-shore ecosystems. If kept in check by herbivory and clean water, Ulva rigida is an important primary producer in intertidal ecosystems. However, when nutrient levels increase, often due to pollution, Ulva rigida can grow to nuisance levels and form macroalgal blooms referred to as "green tides."

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Dawes, C.J. & Mathieson, A.C. (2008). The seaweeds of Florida. pp. [i]- viii, [1]-591, [592], pls I-LI. Gainesville, Florida: University Press of Florida.

Hayden, H.S. & Waaland, J.R. (2004). A molecular systematic study of Ulva (Ulvaceae, Ulvales) from the northeast Pacific. Phycologia 43: 364-382.

Kraft, G.T. (2007). Algae of Australia. Marine benthic algae of Lord Howe Island and the southern Great Barrier Reef, 1. Green algae. pp. [i-iv], v-vi, 1-347, 110 text-figs; 11 pls. Canberra & Melbourne: Australian Biological Resources Study & CSIRO Publishing.

Loiseaux-de Goër, S. & Noailles, M.-C. (2008). Algues de Roscoff. pp. [1]-215, col. figs. Roscoff: Editions de la Station Biologique de Roscoff.

Loughnane, C.J., McIvor, L.M., Rindi, F., Stengel, D.B. & Guiry, M.D. (2008). Morphology, rbcL phylogeny and distribution of distromatic Ulva (Ulvophyceae, Chlorophyta) in Ireland and southern Britain. Phycologia 47: 416-429.

Norris, J.N. (2010). Marine algae of the Northern Gulf of California: Chlorophyta and Phaeophyceae. Smithsonian Contributions to Botany 94: i-x, 1-276.

Pedroche, F.F., Silva, P.C., Aguilar-Rosas, L.E., Dreckmann, K.M. & Aguilar-Rosas, R. (2005). Catálogo de las algas marinas bentónicas del Pacífico de México. I. Chlorophycota. pp. i-viii, 17-146. Ensenada, México: Universidad Autónoma de Baja California.

Smith, G.M. (1944). Marine algae of the Monterey Peninsula. pp. i-ix, 1-622, 98 pls. Stanford: Stanford University Press.

Type locality: Cádiz, Spain (Silva, Basson & Moe 1996: 750). Type: LD herb. alg. Agardh, 14449 (Ricker 1987: 41). Notes: Silva et al. recommend consulting Papenfuss (1960: 305) for further information regarding the type locality of this species. The type locality was given as Cape of Good Hope by Ricker (1987: 41) and he records that a lectotype has been selected by R.B. Searles on 10 October 1975. Womersley (1984: 144) gives the herbarium no. 14294.